An in-wheel motor driving device typically has their drivetrain connected directly to a component near a tire, and because of this, the device is subject to a bending moment caused by a lateral force, such as a turning load from the tire, exerted onto an output gear of the speed reducer.
This bending moment can move a gear shaft out of alignment, which in turn will affect gear meshing operation, which then may result in uneven gear contact, leading to such problems as abnormal noise and wear.
In an attempt to eliminate these problems, a conventional solution is to provide a large mesh clearance between the teeth so that there will not be abnormal interference even if the gear shaft comes out of alignment (Patent Literature 1).